Most people are aware that in
species such as Leopard Geckos, there is a
strong effect of incubation temperature on the sex of the
hatchlings, but most people seem pretty unsure about what
effects it may or may not have on bearded dragons.
Frequently, there are dragons up for sale in various reptile
adverts and for sale sections, that are "incubated for female" - but
when asked, the breeder doesn't actually know how to
"incubate for female", and there is no consistent
temperature given between various breeders who claim to
incubate for female. This article will hopefully explain in
some detail the science behind "incubating for" and explain
why - under normal conditions - this isn't possible for
beardies.

Apologies if some of this is a little bit too "sciency" -
there will be a couple of "concluding" paragraphs at the
end, for anyone who just wants the information, and not the
background stuff etc.

In mammals (and humans) there are two different types "sex
chromosomes" - X and Y. You get one from your mother, one
from your father. If you get XX you are a girl (XX is known
as the "homogametic" combination, as the sex chromosomes are
of the same type). If you get XY, you are a boy (aka
"heterogametic").

Many reptiles and birds do it differently. In bearded
dragons, the chromosomes are called Z and W, and it is the
opposite way round. ZZ (homogametic) gives a "genetic" male,
and ZW (heterogametic) gives a "genetic" female. The reasons
for putting "genetic" in inverted commas will become
apparent later.

The University of Canberra research team first identified
the sex chromosomes - until 2005, it was known that bearded
dragons had genetically-determined gender, but no-one had
managed to identify the sex chromosomes, because they were
too small. Once they could be identified, and the Z and W
could be distinguished, it was possible to check whether a
bearded dragon had a "phenotypic" (physical) gender (hemipenes,
ovaries etc) that
matched up to its "genetic" gender.

It is now believed that the Z chromosome carries a gene
which is crucial for male development. It probably encodes
an enzyme, which catalyses a reaction to produce something
that causes "maleness". This is a similar situation to
humans: by default, humans develop as females, and require
a gene from the Y chromosome (known as "sry") to develop
testes, which then release hormones causing development into
a male. In the absence of this gene, we follow a "default"
pathway of female embryonic development.

The reason that the Z-chromosome gene probably encodes an
enzyme is that it appears to be temperature-sensitive, with
an optimal temperature range, believed to be around 21c
(70f) - 34c (93f). Within this range, it functions as
normal.

In order to become physically "male", the embryo needs a
high dose of whatever the end-product of the enzyme reaction
is. I will refer to is as "sex-determining factor" - SDF -
from now on for convenience. There is a threshold value for this - above
threshold, and you get a male; below threshold and you get a
female as that is the default developmental pathway. A ZZ
male has two copies of the gene; a ZW female has only a
single copy. This means that the dosage to the male will
potentially be
doubled. A genetic female will never receive enough of the
product from the single Z chromosome to develop as a
physical male, whereas under normal circumstances a genetic
male - with two copies of the gene - will receive enough to
develop as a male. However, under some conditions it is possible for a genetic male to
not receive enough of the SDF to develop as a male. In these cases the embryo will follow the
"default" pathway, and
become female.

Genetic Z-chromosome defects aside - which are often lethal
anyway - the only way for a genetic male to not receive
enough SDF is if the enzyme that catalyses its production is
working suboptimally - i.e. outside of its optimal
temperature range. This means that the amount of SDF
produced is below threshold, so although genetically the
embryo is male, it will develop as a female, with female
reproductive organs etc.

Eggs incubated above 93f (the upper end of the enzyme's
optimal activity range) will occasionally not receive
sufficient SDF to develop as males. Above 36c (97f), the
research team found that 100% of hatchlings were physically
female, with a gradual change of percentage physical females
in between. There was a high amount of mortality
at that temperature, due to the incubation temperature being
so extreme. They tried analysing the results assuming that
all mortalities were "genetic, physical males", and still
found that significantly more than 50% of eggs would have
contained females. Of the physical "females" that hatched,
51% were found to be genetically male. They had proved
temperature-dependent sex reversal (TDSR) was possible in
bearded dragons, for high temperatures. As the enzyme's
optimal range is believed to have 21c as a low-end of the
range, they have not been able to prove any form of TDSR for
low temperatures - no eggs incubated below 22c hatched, so
there were no babies to analyse.

If you have a genetic male trying to produce eggs, there may
be meiotic failures due to chromosomes not lining up
properly, which would cause infertility. This may not be the
case, as ZZ chromosomes can line up during sperm production.

If viable eggs were to be produced, they would all be
genetically male - both parents are ZZ, so all babies would
be as well. Unless the eggs were incubated at very high
temperatures to ensure TDSR, all babies would be genetic and
physiological males.

1. It is possible to "incubate
for female", but at much higher temperatures that those
commonly used for incubation. I have never found anyone who
would incubate beardie eggs above
97f (indeed, I've never met anyone incubating above 90f, due
to the increased risk of birth defects and in-egg
mortalities...) At 97f, there are very high mortality rates,
and birth defects are not uncommon, due to other,
non-sex-determining enzymes being affected by the high
temperatures. TDSR females can apparently start to appear from around
93f.

2. The key temperature-sensitive phase of incubation appears
to be the
middle third of egg development.

3. Any "male" eggs which produce physical "females" due to
incubation temperature should theoretically have
reproductive problems - either egg non-viability, or 100%
male offspring. As the research is less than a year old, I
don't think they have had chance to test these theoretical
ideas - it may be that more papers are published over the
next couple of years, when the team gets the chance to grow
some TDSR "females" to breeding size, and breed them with
normal males, to see what offspring (if any) are produced.

4. There have been reports of so-called "hot" female
bearded
dragons. This is a well-known phenomenon in some female
leopard geckos which have been incubated at a high
temperature (intended to produce males). I have written
another article looking at this: Temperature-dependent Sex
Determination in Leopard Geckos. The
leopard
gecko mechanism predicts an increased sensitivity to
testosterone caused by increased egg incubation temperature.
If a similar effect was present in bearded dragons, this
would explain aggressive, stereotypical "male" behaviour by
"hot" females.

This means that whilst it is possible to "incubate for
female" in a lab setting, a lot of the eggs will fail, and a
lot of the hatchlings will have problems associated with too
high an incubation temperature, so it is a dangerous and
(given the theoretical infertility / 100% male offspring
problems) ill-advised strategy. Additionally, many of the
TDSR "females" produced may have behaviour problems that
would make them entirely unsuitable for breeding / living
with other females. This pretty much defeats the point of
attempting to produce more females in the first place.

** Thus babies advertised as "incubated for
female" will almost certainly have the usual 50:50 chance of
being male or female, unless the breeder risks high
mortality and birth defects by incubating at an extreme
temperature. **

As this is fairly recent research (2007), at the time of
writing there has not been sufficient time for the team to
grow on the TDSR "female" babies, to see whether they are
able to produce viable eggs, and whether all of those eggs
would turn out to be genetic males as predicted.